The present inventive subject matter relates to the use of electric fields to guide fish and other aquatic animals and to prevent injury thereof.
The effect of electric currents on fish are well known in the prior art of electrofishing. Electrofishing involves the use of electric currents to attract and/or repel fish with the intent of creating aquatic barriers and/or to improve sample yields during fish conservation activities.
Electrofishing has traditionally been used in freshwater lakes and streams and is the subject of U.S. Pat. Nos. 5,327,854; 4,750,451; 4,672,967; 4,713,315; 5,111,379; 5,233,782; 5,270,912; 5,305,711; 5,311,694; 5,327,668; 5,341,764; 5,551,377; and 6,978,734 which are incorporated herein by reference. Also, electrofishing has been the used to stimulate yields of trawl net fishing as described in U.S. Pat. Nos. 3,110,978 and 4,417,301 which are also incorporated herein by reference.
It is well known in the prior art that relatively small potentials impressed across the body of a fish invoke a flight reaction. Larger potentials result in the alignment of the fish with the electric current. Still larger potentials may result in electronarcosis and/or the complete euthanasia of the fish. (See Introduction to Electrofishing, pages 24-26, Smith-Root, Inc. which is incorporated herein by reference).
As a consequence of the well-established physiological reaction of fish to electric currents, it is possible to construct “electric barriers” in water that are designed to deter fish. The underwater electric barrier can be thought of as the above ground analog of the electric fence commonly used to constrain livestock in a field. Although the above ground analogy to underwater barriers is easy to conceptualize, the fact that water is a conductive media creates a number of additional technical considerations in the construction of electrified barriers. For example, the placement of electrodes, cost of power consumption, and the potential harm to endangered species are all considerations for underwater barriers that are generally not found in above ground electric barriers.
Despite the inherent technical challenges posed by underwater barriers, underwater electric barrier technology for fish entrainment has been used in a number of locations both nationally and internationally. Representative examples of such barrier systems that are used to guide fish. Representative barriers are manufactured by Smith-Root, Inc. are located at the Granite Reef Dam, on the Salt River (Arizona, U.S.A.), the Chicago Sanitary and Ship Canal (Illinois, U.S.A.), and Eagle Creek National Fish Hatchery (Oregon, U.S.A.). Exemplary barrier systems are also illustrated in U.S. Pat. No. 5,445,111 (Aug. 29, 1995) issued to David Smith which is incorporated herein by reference.
Despite the wide diversity of geographic places in which these barrier systems are deployed they operate in a similar manner. These systems typically consist of a: 1) a pulsator unit, 2) an electrode array, and 3) a mechanical structure in which the electrode array is attached (e.g. electrode array support structure). (See Prior Art FIG. 1). The pulsator unit provides the electrical potential in either AC, DC, and/or Pulsed DC waveforms. The electrode array is immersed in the water and the electrode array support structure fixes the electrodes in a predetermined spaced array. When the pulsator unit energizes the electrode array, an electric field is created in and around the spaced array of fixed electrodes. The potential field can be calculated based on the electric potential of the pulsator and the location of each electrode in the spaced array.
In a typical configuration for a barrier operation the pulsator is designed to generate electric fields at all times. This basis for this continuous operation is to insure that fish are repelled, by the barrier at any time. As previously discussed, these electric fields may be AC, DC, and/or pulsed DC. Unfortunately, the continuous operation of the electrical barrier results in higher operational costs due to energy costs and increased maintenance.
For example, a continuously operating barrier will draw electricity continuously even if no fish are present. This may be inefficient during periods of time where there is no passage of fish. Furthermore, all electrodes experience degradation as a consequence of the electrolytic action of the electrode with the water. Therefore a continuously operating barrier will cause maximum amount of electrolytic action of the electrodes and consequently, increased degradation of the electrodes.
Also, it is not uncommon for electrical fish barriers to be turned on and off for maintenance purposes. Likewise, there may be situations where electrical fish barriers may be turned off in response to certain situations, for example, in the presence of a non-target object or organism.
One of the problems of electrical fish barrier “turn-on” and/or pulsator initiation, is the undesired side effect that may occur for certain bottom dwelling species of fish (e.g., sturgeon) that may be located near the electrode array or perhaps nestled against an electrode. The application of an operational voltage by the pulsator to electrode array may result in immediate electrotaxis and/or physiological damage to the fish due to an electrically induced convulsive response. Some of these fish are commercially valuable and thus any damage due to the effects of the electrical barrier being energized may diminish their potential catch value.
Therefore, in an effort to ameliorate the effect of a large electrical potential being applied to a fish barrier, what is desired is an apparatus and method to slowly “ramp up” the electrical field with a goal to minimize the damage caused by existing pulsators or barrier systems on benthic species.